Flotation consists in extracting minerals out of suspensions of ores, generally aqueous suspensions, by rendering more hydrophobic (less wettable by water) the particles to be floated, using reagents, usually referred to as “collectors”. Direct flotation process refers to processes where the floated particles are the ores of value, whereas reverse flotation process refers to processes where the floated particles are the impurities to be extracted out of the ores of value.
Flotation process generally takes place in a cell containing an aqueous suspension of ores to be treated, and a generator of air bubbles. At least one collector is added and the at least one collector adsorbs onto the surface of the particles of minerals or impurities to be removed (case of the reverse flotation), enhancing the attachment of the particles with air bubbles upon collision. The combined air bubbles/particles, less dense than the pulp, go up to the surface, leading to the formation of a froth, which is collected by skimming or via an overflow.
Mineral flotation such as flotation of silica, silicates, feldspath, mica, clays, potash and other minerals, which bear a negative charge at the pH value where the flotation is operated, is typically achieved by using cationic collectors. Cationic collectors are molecules that are at least partly positively charged when added in an aqueous environment at an appropriate pH value.
Hence, the term “cationic collectors” is herein understood to represent organic collector compounds containing at least one amino group. Such cationic collectors are already known, widely used, and include e.g. fatty amines and their salts, fatty propylene polyamines and their salts, alkyl ether amines and alkyl ether diamines and their salts, quaternary ammonium salts, imidazoline derivatives, alkoxylated amines, and the like.
The use of quaternary ammonium compounds as collectors in reverse froth flotation processes for example for calcite ores has long been known. The meaning of the term “reverse froth flotation” is that the froth is used for carrying the gangue mineral rather than carrying the valuable concentrate, i.e. the gangue is recovered in the froth product. See for example U.S. Pat. No. 4,995,965, where calcium carbonate and impurities such as silicate, are separated by floating the silicate and concentrating the calcium carbonate in the remainder, in the presence of collectors such as dialkyl dimethyl quaternary ammonium compounds.
However, dialkyl quaternary products which are currently used for reversed flotation of calcite, for example such as those described in U.S. Pat. No. 4,995,965, have the drawback of being toxic for aqueous organisms and are also regarded as being not readily biodegradable in environment.
DE19602856 proposes to use biodegradable ester quats as collectors in a reverse froth flotation process. These products are quaternary fatty acid alkanolamine ester salts. However, such ester quats were found to degrade, by hydrolysis and/or biologically during the flotation step, releasing fatty acid, particularly in the typical process where the aqueous phase is recycled. In the calcite reverse froth flotation process there is a risk that the fatty acid released may attach to the calcite and float the mineral, resulting in poor yields.
Recently a new class of polymeric ester quats, such as products obtainable by reacting alkanolamines with a mixture of monocarboxylic acids and dicarboxylic acids, has been proposed in international application WO2008/089906.
These products meet the demand of being nontoxic, readily biodegradable products that seem to be sufficiently efficient in flotation. However, they exhibit the same disadvantage as the ester quats mentioned above, with fast release of fatty acid upon hydrolysis, especially when used in a flotation process environment of high pH (around 10) and an elevated temperature (above 30° C.). The release of fatty acid soap can possess a risk as this substance has the opposite collecting properties to the ester quats, thus supporting flotation of the valuable calcite which is then going to waste.
To solve this issue, international application WO2011/147855 proposes another class of oligomeric esterquats. The condensation products described in this patent are represented by the general formula:
wherein R1 is a hydrocarbyl group having 7-24 carbon atoms, which may be branched or linear, saturated or unsaturated, AO is an alkyleneoxy group having 2-4 carbon atoms, n is a number between 0 and 20, Y is O, C(═O)NH or NZ, where Z is a group R2, where R2 is a C1-C4 alkyl group, preferably CH3, or the benzyl group; provided that when Y is NZ or C(═O)NH, then n is greater than or equal to 1; R3 is an alkylene radical of formula —(CH2)Z—, in which z is an integer from 0 to 10, preferably from 2 to 4, and most preferably 4, and in which the alkylene radical may be substituted by 1 or 2 —OH groups, the group —CH═CH—, a cycloalkylene, a cycloalkenylene or an arylene group; each x independently is a number between 1 and 5, and the sum of all x on average is a number between 2 and 10; R5 is a C1-C3 alkyl group or a group [AO]x; t is 0 when Y is O or C(═O)NH, and t is 1 when Y is NZ; R6 is a hydrocarbyl group, preferably a C1-C4 alkyl group or the benzyl group, and X− is an anion derived from the alkylating agent R6X; and p is typically a number within the range 1-15, and is on average at least 1, preferably at least 2 and most preferably at least 3; the average value of p depending on the molar ratios of the different compounds used in the reaction mixture, as well as on the reaction conditions.
These products are claimed to avoid release of fatty acid upon hydrolysis. They also present an improved resistance to hydrolysis in a flotation process environment of high pH (around 10) and an elevated temperature (above 30° C.). Anyway, as will be presented in Comparison Example 9 of the present invention, there is still room for improvement concerning the performance of this type of valuable collectors and especially when it comes to resistance to hydrolysis in harsh flotation conditions (high pH and high temperature).
Hence there is a continued need to optimize and/or find alternatives for the reverse froth flotation process of calcium carbonate ores. In this respect it is particularly important that the amount of acid-insoluble material in the product is as low as possible, the yield of product is as high as possible, and that a product of high quality (particularly brightness) is obtained. Due to the huge amount of ore treated per day, it is of particular interest to reach this goal with a dosage of collector as low as possible. It should be realized that reducing the amount of acid-insoluble material and increasing the yield are two mutually conflicting goals.
More specifically, reducing the amount of acid-insoluble material is typically achieved by floating off a large amount of material, but this reduces the yield of the overall beneficiation process, and vice versa.
Apart from calcite ores, there are other valuable ores that contains silicates as impurities and where highly efficient cationic collectors are of great interest. Phosphate and iron ores beneficiation are other examples of situations where this type of collector can be particularly valuable.